Extreme heat events lead to considerable health burden and are becoming more severe and frequent, calling for the development of effective population-based and individualised heat early warning systems. We developed an individualised heat early warning system and tested it in 78 older adults' ( ≥ 65 years) homes in Southeast Queensland, Australia. Quantitative and qualitative data from this proof-of-concept testing study showed that the Ethos system performed well on a standard usability scale (mean score of 78 on the System Usability Scale). Following a summer-time use of this early warning system, there were increases in heat preparedness (P <
0.001, marginal homogeneity tests) but no significant increases in heat health risk perception or the uptake of low-cost cooling measures (e.g., hand/forearm bath, fans). This proof-of-concept research demonstrated the usability of this tailored, actionable, real-time digital heat early warning system, although the effectiveness of the system remains to be evaluated in a robust trial design.